Dear A/B, regarding 'rifle' as such...the firearm is so-called because its barrel has a groove or 'rifle' cut into it in order to impart spin to the bullet. That version is related to the word 'riffel', meaning 'groove' in Germanic/Scandinavian languages. In the sense of 'plunder' the word relates to the Old French verb 'rifler', meaning to 'scrape' or 'strip'...ie to remove. (The ideas of scraping and grooving are, of course, related, since both involve removing something.) Most similar word-pairings of the sort can be explained in the same sort of way; namely, that they entered English via different 'doors'.